Using Everyday Examples in Engineering (E 3 ) Cooking with Microwaves

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1 Using Everyday Examples in Engineering (E 3 ) Cooking with Microwaves Peter Y. Wong Tufts University Introduction Students are very familiar with using microwave ovens to make popcorn or to reheat leftovers. In this class, they will learn more about cooking with microwaves and how it relates to heat transfer and thermodynamics. Engage the students with either physical props (e.g., microwave and marshmallow experiments) or some multimedia videos (i.e., there are many experiments with microwave ovens online). Follow up with a classroom discussion about some of their experiences with using a microwave. What things should not be put into a microwave? Many students will answer with metal objects. Show them the packaging in a commercial food product that crisps food. There is metal in there. Why is it necessary to rotate the food every once in a while? Students will respond with even heating. Ask why is the heating uneven. Where are the microwaves coming from? Is it safe to look at the food in the microwave? Students might say yes, but my mother told me not to look too long. Why does light pass through but not the microwaves? Do foods heat faster in microwaves than toaster ovens? Students may say It depends and then add examples such as toast and pizza for toaster ovens and cups of water and leftover chicken for microwaves. What is the difference between the two kitchen appliances? These questions can help engage the classroom to re-examine their thinking about how the food gets heated and how microwave ovens work. This material is based upon work supported by the National Science Foundation (NSF) under Grant No Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of NSF.

2 Background on Microwaves Microwaves are part of the electromagnetic spectrum and are used for many technologies including communication, radar, and food processing. The electromagnetic spectrum consists of various types of radiation, characterized by wavelength (λ) and frequency (f). The frequency of the wave can be visualized as the number of wave crests that move by an observer in one second (Figure 1). Microwave radiation refers to the region of the spectrum with frequencies between ~10 9 Hz to ~10 11 Hz. This type of radiation lies between infrared radiation and radio waves. Figure 1. How to measure the frequency of a wave. In the case shown here, the frequency of the wave is 2 Hz, or 2 cycles per second. Waves with shorter wavelength (higher frequency) have higher energy. The frequency of visible (orange) light at λ = 600 nm is approximately 5x10 14 Hz. Toaster ovens with either quartz bulbs or resistively heated elements shine with a bright orange and heat food surfaces by thermal radiation. The frequency of radiation used in microwave ovens is approximately 2.45x10 9 Hz (2.45 GHz), with a wavelength of cm. This means that the microwave radiation used to heat food is actually less energetic than visible light. Why, then, are microwave ovens considered so efficient at heating food? The answer is related to the interaction of that wavelength with food- in particular water molecules. In fact, the interaction was discovered by accident. If you have time, you may talk about the invention of the microwave cooking. An engineer discovered that his chocolate bar melted in his pocket while working with RADAR technologies. There are other examples of technologies developed by accidental discoveries. Some examples are hook-and-loop fasteners, penicillin, saccharin, Teflon coated pans, and vulcanized rubber. This is also an opportunity to discuss the relation between science and engineering.

3 Water Molecules and Microwaves Water molecules (H 2 O) are polar; that is, the electric charges on the molecules are not symmetric (Figure 2). The alignment and the charges on the atoms are such that the hydrogen side of the molecule has a positive (+) charge, and the oxygen side has a negative (-) charge. Figure 2. Polar nature of water molecule Electromagnetic radiation have electric charge as well; the "wave" representation shown in Figure 1 is actually the electric charge on the wave as it flips between positive and negative. For a microwave oscillating at 2.45x10 9 Hz, the charge changes signs nearly 5 billion times a second (2 times 2.45x10 9 Hz). Electric charges react similarly to how magnetic charges do - opposites attract. When oscillating electric charge from radiation interacts with a water molecule, it causes the molecule to flip (Figure 3). Microwave radiation used in ovens take advantage of the water molecules flipping characteristic along with the flipping dissipation due to other neighboring water molecules. The dissipated energy is in the form of heat which can be inferred from a temperature measurement rise when the water is in liquid state. Figure 3. Interaction between microwave and water molecule So microwaves work well with foods that have water in them. Microwaves also work with fats and sugars, but not as efficiently. Interestingly, when water is solid (i.e., ice) then the microwaves do not flip the water molecules and there is no dissipation or heat addition to the ice. So the defrost cycle on microwaves are designed to heat some small amount of water on the frozen food in a short period (on the order seconds), wait for that heat is be conducted through the food (a few seconds again), and then repeat until enough of the water has melted. Steam also has a weak interaction with microwaves.

4 Inside a Microwave Oven Microwaves are produced by a device called the magnetron which consists of four major components: (1) anode block (hollow cylinder with vanes pointed into the center), (2) cathode filament, (3) pair of permanent magnets, and (4) antenna. The top view of the anode block is shown in Figure 4. The spaces between the vanes are resonant cavities. The cathode filament is a cylindrical rod located at the center of the anode block. It serves as the source for electrons during emission of microwave radiation. Two permanent magnets are located at the top and bottom of the anode block, as shown in the side view in Figure 5. These magnets create a magnetic field inside the anode block that is parallel to the cathode filament. The antenna is positioned so that it starts in a resonant cavity and terminates in the waveguide. The waveguide transfers the microwaves to the cooking chamber, much like how fiber optics are used to transfer light. Figure 4. Top view of anode block Figure 5. Side view of the magnetron The events that lead to the production of microwaves begin when the cathode emits electrons. Since the filament and the electron both have negative charge, electrons accelerate outward, toward the vanes. The changing electric field from the electrons induces a magnetic field around itself. This magnetic field interacts with the magnetic field from the permanent magnets. The interaction of a moving electric charge in a magnetic field results in a net force. In this case, the magnetron has been designed so that electrons move clockwise around the cathode filament, forming a cloud (Figure 6). Figure 6. Electron motion in anode block

5 As an electron approaches a vane, a positive charge is induced within the vane as electrons are repelled. Meanwhile, a negative charge is induced in a neighboring vane due to an accumulation of repelled electrons. Negatively charged vanes then repel electrons rotating within the cloud forming a pinwheel shape (Figure 7). Vanes closest to the "spokes" have a positive charge, and neighboring ones are negative. As the pinwheel rotates, vane charge oscillates from positive to negative creating an alternating current within resonant cavities. The antenna carries current to waveguide. Figure 7. Electron cloud and induced currents The final step in a microwave oven is to release the radiation within the cooking chamber. When radiation exits the waveguide, it is often reflected by a stirrer blade to evenly distribute radiation throughout the cooking chamber (Figure 8). Once entering the chamber, the radiation is reflected by the chamber walls until it is absorbed by the food. However, the spreading is not even throughout the chamber. Figure 8. Microwave chamber A drawback in microwave ovens are hot and cold spots. Waves reflecting around the chamber interact with each other through a phenomenon called interference (Figure 9). Waves overlapping so that crests match one another interfere constructively, forming a hot spot. Waves interfering destructively result in a cold spot. Figure 9. Interference of waves

6 Cooking with a Microwave Oven. Microwave heats food by agitating the water molecules in liquid state. Most foods we consume contain over 70% water by weight, making this an effective way for heating foods. However, the negative side is that food with low water contents take longer to heat in a microwave. There are also hot spots and cold spots to consider in cooking large pieces of food. As mentioned previously, frozen foods take longer to heat because the water molecules in solid state are not moved as much as in liquid water. Ask students to graph the power duty cycle for a microwave in the defrost cycle. If you have an actual microwave in the classroom or lab, the students can make observation (especially by listening to the microwave) to determine the duty cycle. At the other extreme, foods heated in a microwave cannot become hotter than the boiling point of water, or 100 C. After water turns to steam, there is a large drop off in interaction between the 2.45 GHz frequency and steam. In fact, the preferred frequency for interaction jumps to 22 GHz. Thus, foods typically cannot be browned in a microwave since browning occurs at temperatures well over 100 C. This can also be observed with reheated pies that do not have a crisp crust like a conventional-oven baked pie would have. A specially designed metal susceptor can be used in food packaging to allow for browning. Do microwaves cook foods from inside to outside? No, but food in a microwave oven is heated much faster than in a conventional oven. Microwave radiation has the ability to penetrate several centimeters into the food similar to how radio waves can go through concrete walls or deep into tunnels (up to a point) GHz microwaves penetrate into food several centimeters down and can heat quicker than through conduction alone. If the food were larger than several centimeters, the middle of the food still needs to be heated by conduction. So some microwaves will also go through a duty cycle to allow the heat to spread through the food via thermal conduction. If marshmallows are placed throughout a microwave oven, there will be some a visible indicator of the uneven heating (hot spots and cold spots) in microwaves. The hot spots will have marshmallows enlarging more rapidly due to the heating of the water (and then the air) in the marshmallows. This uneven heating can be reduced by using a rotary platform; as well as stirring the food every few minutes. You can watch the cooking of the foods in a microwave; although it is through a dim window. Looking closer at the window will review a grid like panel that prevents the microwaves from leaving the cooking chamber. This works because the spacing in the grid (which is metal) is much less than the wavelength of the microwave radiation (12.2 cm). The grid does allow visible light ( nm) to pass through since the wavelength is much smaller than the grid openings. Students can investigate this shielding by searching for Faraday cages also.

7 Evaluation Here are some end-of-class questions to pose to students to help gauge their understanding of the class content: 1) What are three advantages of microwave ovens over conventional ones? 2) What are three disadvantages to microwave ovens? 3) What technology developments have helped to overcome some of the disadvantages? Possible homework problems: 1) Explain in detail the interaction of microwaves and an ice cube that eventually becomes saturated vapor. 2) Compare a conventional oven recipe with a similar microwave-suitable one (e.g., cooking a cake). 3) Research whether or not food cooked in a microwave is germ-free. Useful Links How Microwave Ovens Work by HowStuffWorks.com An interactive tutorial on Microwave Ovens from University of Colorado, Physics 2000 project. Invention of the Microwave Oven from American Heritage References Incropera, F.P. and De Witt, D.P., Introduction to Heat Transfer, Second Edition, John Wiley & Sons, New York, NY, McGee, H., On Food and Cooking: The Science and Lore of the Kitchen, Simon and Schuster, New York, NY, Horn, J., Cooking A to Z: The Complete Culinary Reference Source, New and Revised Edition, Cole Group, Santa Rosa, CA, Wolke, R.L., What Einstein Told His Cook, W.W. Norton & Company, New York, Peter Wong. All rights reserved. Copies may be downloaded from This material may be reproduced for educational purposes.